Sub-natural absorption resonances on the D1 line of rubidium induced by coherent population trapping

A narrow (70 kHz fwhm) high-contrast resonance formed due to coherent population trapping (Λ-resonance) has been observed in 87Rb vapour by using two independent laser diodes. The first results on the dependences of its width on magnetic field, laser intensity and divergence are reported. The Λ-resonance is used to stabilize the difference of laser frequencies to the hyperfine splitting of the Rb ground state. A new technique of stable microwave signal generation is proposed. It is based on optical locking of two lasers via Λ-resonance in a cell inserted in an external cavity of a laser diode